docs: def _spectrum(self, photon_energy):
          """Compute differential bremsstrahlung spectrum for energies in
          ``photon_energy``.
  
          Parameters
          ----------
          photon_energy : :class:`~astropy.units.Quantity` instance
              Photon energy array.
          """
  
          Eph = _validate_ene(photon_energy)
  
          spec = self.n0 * (
              self.weight_ee * self._emiss_ee(Eph)
              + self.weight_ep * self._emiss_ep(Eph)
          )
  
          return spec